1. Field of the Invention
This invention relates to an optical transmission device for the optical communication system or an optical transmission/receiving device combined with a receiving device.
This application claims the priority of Japanese Patent Application No.2000-12857 filed on Jan. 21, 2000 which is incorporated herein by reference.
This invention aims at an improvement of an inspection part of the light emitting device for monitoring the power of the light emitting device. The present invention can be widely applied to the communication systems making use of optical fibers as a medium. The optical signal transmission device includes an LD (laser diode) as a light emitting device. The LD power varies due to the change of temperature or the degradation by aging. The light source device has, in general, a PD (photodiode) for detecting the power of the LD and a controlling device for adjusting the driving current by feedback of the PD signal and for maintaining the output power of the LD. The PD is called a xe2x80x9cmonitoring PDxe2x80x9d. This invention proposes an improvement for the coupling of the monitoring PD with the object LD.
2. Description of Related Art
An LD module which is employed for transmitting optical signals of optical communication systems is described by referring to FIG. 1. The LD module 1 has a metallic round stem 2 with an erect mount 3. An LD chip 4 is fixed on a side of the mount 3. The LD chip 4 emits light in upward and downward directions at a certain rate. A monitoring PD chip 5 is fixed at a center of the stem 2 beneath the LD chip 4. A metal cap 6 with an opening 7 covers the LD 4, the PD 5 and the mount 3 on the stem 2. The foot of the metal cap 6 is welded on the stem 2. The light emitted upward from the LD 4 passes through the opening 7 of the cap 6. A cylindrical metallic lens holder 8 having an opening is welded upon the stem 2. The lens holder 8 supports a lens 9 at the opening. A metallic conical ferrule holder 10 is welded upon the lens holder 8.
An optical fiber 11 which carries optical signals is held by a ferrule 12 at its end. The axial hole of the ferrule holder 10 seizes the ferrule 12. Pins 13 downward project from the bottom of the stem 2. In the assembling steps, the optimum position of the lens holder 8 is determined by displacing the lens holder 8 in the xy-plane, measuring the light power at the other end of the fiber, and seeking the spot which brings about the maximum power to the fiber. The optimum position of the fiber is determined by displacing the fiber in the axial direction (z-direction), measuring the power at the other end of the fiber and fixing the fiber at the spot which maximizes the power. The operation for seeking the optimum positions of the lens holder and the fiber is called xe2x80x9calignmentxe2x80x9d.
This invention pays attention to the relation between the monitoring PD 5 and the LD 4. The monitoring PD 5 laid behind the LD 4 always monitors the rear light of the LD 4. Thus, the monitoring PD 5 can detect the change of the rear light power of the LD 4. The front light of the LD 4 is signal light which carries signals to another terminal. The front light of the LD 4 is in proportion to the rear light. The power of the LD can be maintained at a constant level by regulating the driving current for cancelling the long-term change of the laser power level obtained by the monitoring PD.
In the example, the LD chip 4 emits light in the z-direction vertical to the stem plane (xy-plane). The LD light (z-direction) is orthogonal to the plane (xy-plane) of the package (stem 2). Thus, the structure of the LD module is three-dimensional. The rear LD light shoots the top of the PD chip 5. The PD is a top surface-incidence type. The PD 5 receives almost all of the LD rear beam. The PD 5 can gather the rear beam at high efficiency due to the three-dimensional structure. The PD 5 can obtain the strong LD rear light in the arrangement. Conveniently, this type allows the PD 5 to lie directly upon the stem 2. Since the package is made from metal, the LD module has strong points of high seal performance and low-noise property. The LD module of FIG. 1 has some advantages such as the strong monitoring light, the low-noise and the tight hermetic sealing. The current optical communication employs this metal-can type LD module as a signal transmission device.
The three-dimensional LD module is still expensive due to the high cost of the parts and the high manufacturing cost. The direction of the beam emitted from the LD is upward, i.e., vertical to the stem plane. When the LD module is mounted upon a print circuit board at the bottom pins 13, the cylindrical metal package is so tall that the LD module hinders efficient arrangement of the circuit boards in an apparatus.
Since the current LD modules have these difficulties, new two-dimensional, planar type LD modules have been intensively studied. The new planar type LD module determines the light path on a surface of a substrate and arranges devices on the surface of the substrate in two-dimensional arrangement. Since the devices and the paths are arranged on the plane, the type of the modules is called a xe2x80x9cplanar lightwave circuit (PLC)xe2x80x9d. All the light paths and all the devices lie on the surface of the substrate in the PLC modules. Although the light path extends in the z-direction in prior modules, the light paths lie on the xy-plane in the planar type devices. Various kinds of PLC modules have been proposed. FIG. 2 shows an example of a planar lightguide type LD module. A silicon (Si) substrate 14 is placed upon a package 15. A laser diode (LD) chip 16 which makes transmission signals is mounted upside down (epi-down) upon the Si substrate 14. A lightwaveguide 17 is formed along a center line on a forward half region of the Si substrate 14. A flat submount 18 having a PD 19 on the front surface is erected upon the package 15. The PD 19 is provided by mounting the PD 19 on the surface of the submount 18 and sticking the side of the submount 18 on the package 15. The PD 19 is a monitoring PD for sensing the rear light beam of the LD 16.
The lightwaveguide 17 is described by referring to FIG. 3 which is a vertically sectional view of a part of the lightwaveguide 17 and the Si substrate 14. An undercladding layer 24 of SiO2 and a linear core 21 and an overcladding layer 25 of SiO2 are formed on the silicon substrate (Si-bench) 14 by the sputtering or the CVD. The linear core 21 has a refractive index higher than the refractive index of the cladding layers 24 and 25. The core 21 is a SiO2 part doped with a dopant which raises the refractive index, e.g., germanium (Ge). The lightwaveguide is fabricated by making the undercladding SiO2 layer 24 and the Ge-doped SiO2 layer 21 by, e.g., sputtering, etching unnecessary sides of the Ge-doped SiO2 layer away by lithography and piling the overcladding layer 25 on the Ge-doped stripe and the undercladding SiO2 layer 24 by sputtering. The striped Ge-doped core 21 is buried in the overcladding SiO2 layer 25. The difference of the refractive indexes enables the core 21 to maintain the propagating light without dissipation. Since the substrate is silicon, the SiO2 layers can be made by a thermal diffusion method instead of sputtering.
The core of the optical fiber 20, the core 21 of the lightwaveguide 17, the light emitting part (stripe) 22 of the LD 16 and the center of a sensing region 23 of the PD 19 lie on the same level. The monitoring PD 19 is a top surface incidence type PD. Since the submount 18 supports the PD 19 on the side, the top of the PD 19 faces the LD 16. The vertical support enables the PD 19 to receive the rear light of the LD with high efficiency. The PD 19 can obtain strong monitoring light from the LD 16. The top incidence type PD is a common PD which can be obtained easily on the market. This PLC type LD module has advantages of the mounting of the fiber and the LD on the same surface, the strong monitoring light and the use of the common PD.
The PLC module of FIG. 2 has still weak points. Since the PD has the light sensing region for receiving the LD rear light at the top, the PD should be set sideways. The submount 18 is indispensable for supporting the PD 19 sideways on the package. The submount 18 lifts the PD at a certain height from the bottom. The Si substrate 14 is necessary for raising the LD at the level of the center of the sensing region 23 of the PD 19 for introducing the LD rear light to the top of the PD 19. Namely, it is impossible for the module to put the PD 19 on the same substrate on which the LD 16 rides. The PD should be set on the package via the extra submount 18, which raises the cost of assembly. The module is not a true PLC, because the base surface of the PD is different from the base plane of the LD. The PD requires an extra operation of the alignment of the PD, which consumes a lot of time. The module of FIG. 2 making use of the top incidence type PD 19 cannot overcome the difficulty of the different heights of the LD and the PD. There are several proposals for solving the problem and for coinciding the height of the PD with the height of the LD by contriving the structure of the monitoring PD.
{circle around (1)} T. Yamamoto, N. Yamamoto, S. Sasaki, M. Norimatsu, K. Tanaka, M. Kobayashi, K. Miura, M. Yano, xe2x80x9cHighly uniform optical power monitor characteristics using surface mounting technology onto PLC platformxe2x80x9d, PROCEEDINGS OF THE 1997 ELECTRONICS SOCIETY CONFERENCE OF IEICE, C-3-97, p206 (1997).
{circle around (2)} Gohji Nakagawa, Seimi Sasaki, Naoki Yamamoto, Kazuhiro Tanaka, Kazunori Miura and Mitsuhiro Yano, xe2x80x9cHigh Power and High Sensitivity PLC Module Using A Novel Corner-illuminated PIN Photodiodexe2x80x9d, 1997 Electronic Components and Technology Conference, p607 (1997), in particular FIG. 10.
FIG. 4 shows the newly proposed LD module having a monitoring PD which is a strange novel PD. A flat Si substrate 26 is fitted upon a package 27. A laser diode (LD) 28 is mounted epi-side down (upside down) upon the Si substrate 26. A PD 29 for monitoring the LD power is mounted at the back of the LD 28 on the Si substrate 26. The monitoring PD 29 has strange bottom ends. A lightwaveguide 30 is made in front of the LD 28 on the Si substrate 26. The lightwaveguide 30 is a straight waveguide or a curved waveguide formed on the Si substrate. The section of the waveguide 30 is similar to the section of FIG. 3. An optical fiber 31 is stuck to the front end of the lightwaveguide 30. A core 36 of the fiber 31, a core 35 of the lightwaveguide 30 and an emitting stripe 36xe2x80x2 are aligned at the same level. The bottom ends of the PD 29 are slantingly ground for introducing the LD back light from the slanting bottom end (angled surface) 33. The slanting cut ends are excellent contrivance for bending the incidence light upward by making use of the high refractive index of the PD substrate. The rear light emitted from the back end of the emission stripe 36xe2x80x2 of the LD 28 shoots the slanting end 33 of the PD 29, bends upward in the PD and arrives at the light receiving region 34 of the PD 29. This strange type PD is called a xe2x80x9cCorner-illuminated PDxe2x80x9d.
The LD front light which is emitted from the front end of the emission stripe 36xe2x80x2 propagates in the core 35 of the light waveguide 30 and enters the core 36 of the fiber 31. The module exploits the merits of the PLC by setting the LD 28 and the PD 29 at the same level on the surface of the substrate 26. However, the module of FIG. 4 has a drawback of the difficulty of making the slanting bottom ends of the monitoring PD 29. Small tolerances of the slanting angle and the length of the edges 33 make it difficult to produce the novel PD. Since the light goes from the end 33 to the PD 29, the light power is weak at the light sensing region 34 of the PD 29. Further, it is not easy to handle the strange type PD. The difficulties of the FIG. 4 module mainly derive from the strangeness of the slanting-cut end PD (corner-illuminated PD) 29.
A third prior art monitoring PD is now clarified. The PD is also one of the end-incidence types which enables the LD light to enter the PD from the end. This is called a waveguide type PD. Another prior art LD module pursues the advantages of the PLC by making use of the light waveguide type PD.
{circle around (3)} M. Shishikura, H. Nakamura, S. Tanaka, Y. Matsuoka, T. Ono, T. Miyazaki, and S. Tsuji, xe2x80x9cA symmetric double-core InGaAlAs waveguide photodiode for hybrid integration on optical platformsxe2x80x9d, LEOS""96, 9th annual meeting, 1996, 18-21, November 1996 (IEEE Laser and Electro-Optics Society 1996 Annual Meeting).
The proposed LD module makes use of an end surface incidence type (waveguide type) PD. FIG. 5 shows the prior LD module which is built on a silicon substrate (bench) 37. The silicon substrate 37 has a flat smooth surface. The silicon substrate 37 is fixed upon a package 38 (via a lead frame). The silicon substrate 37 has an LD chip 39, a monitoring PD 40 along a line upon the top. A light waveguide 41 is formed upon the silicon substrate 37 in front of the LD 39. The core 44 of the waveguide 41, the emission striped 43 and the sensing layer 46 of the PD 40 are aligned on an axial line. The LD 39 emits signal light in the forward direction and monitoring light in the rear direction. The forward signal light from the LD 39 propagates in the light waveguide 41 and the optical fiber 42 to a central station or other terminals. The backward light from the LD 39 shoots the end surface of the monitoring PD 40 and generates photocurrent in the PD which is in proportion to the forward signal light. In the LD module, the emission stripe 43 of the LD 39 is level with the sensing layer (light receiving layer) 46 of the PD 40. The straight alignment enables the LD module to dispense with extra submounts for the PD. This is an ideal configuration of the PLC devices. It seems to propose an excellent monitoring PD. The end surface incidence type PD is new and strange. {circle around (3)} describes the contrivance for making the novel PD. The LD module is surpassing. {circle around (3)} describes the new PD itself. The PD has a far more complex structure than the prior PDs do. The difficult manufacturing and the low yield of the new PD raise the cost of producing the LD module of {circle around (3)}.
The LD modules which try to make use of end-surface incidence type (waveguide type or slanting end incidence type) PDs seem to be endowed with simplified structures. But the novel PDs which allow the monitoring light to enter the PD via the end surface are difficult to produce, unstable in the performance and immature yet. At present, the PDs which can be easily obtained in the market are the top surface incidence type PDs (FIG. 1 and FIG. 2) or the bottom surface incidence type PDs. The present inventors intend to make use of the commonplace, inexpensive and reliable PDs which will promise the low cost, high reliability and high performance of the LD modules built by the PDs.
{circle around (4)} German Patent DE43 13 492 C1, xe2x80x9cAnordnung zur Ankopplung eines optoelektronischen Empfangselementes an ein optoelektronisches Sendeelementxe2x80x9d, inventors; Schwanderere Bernhard, Kuke Albrecht.
As shown in FIG. 20, {circle around (4)} proposes an LD module which monitors the power of the LD by a bottom surface incidence type PD. The LD module is built upon a silicon substrate 200 which has a longitudinal rear groove 204 and a front groove. The path conversion groove 204 has a front slanting wall 201, horizontal walls 202 and a rear slanting wall 203. An LD 205 is mounted upside down (epi-down) at an interposing region between the grooves upon the substrate 200. A bottom incidence type PD 206 is mounted over the elongate groove 204 on the substrate 200 in a direction slightly slanting to the axial line. The bottom incidence type means a PD having a bottom annular n-electrode with a transparent opening which allows signal light to enter the PD via the bottom opening. The light receiving region (sensing layer) 208 is made at the top center of the PD 206. Almost all of the bottom of the PD 206 faces with the elongate groove 204. The bottom of the LD 205 is on the same level as the bottom of the PD 206. The PD 206 dispenses with a submount for raising the PD 206. Since the LD 205 is fixed epi-down upon the substrate 200, the emission stripe 207 of the LD 205 is very close to the surface of the Si-substrate 200. The LD 205 emits signal light in the left direction and monitoring light in the right direction in FIG. 20. The front groove sustains a lens and a fiber. The lens converges the front signal light to the fiber. The rear light matters in the German patent. The monitoring light which is emitted in the right (rear) direction diverges in the vertical direction. The rays L0, L1, L2, . . . of the rear light are depicted in FIG. 20 for showing the divergence of the LD rear light. The downward dispersing rays (209) L1, L2 and L3 are reflected by the rear slanting wall 203 or the horizontal wall 202 and are introduced into the PD 206 via the bottom surface. The rays (L1, L2, L3, . . . ) 209 are refracted upward at the bottom boundary and are guided to the light receiving (sensing) region 208 of the PD 206. The rays generate photocurrent in the monitoring PD 206 in proportional to the LD power. It is possible for the downward emitted rays to arrive at the sensing region and to make the photocurrent in the PD 206. However, the just-horizontally emanating ray L0 211 and the upward emitted rays L4 do not enter the path conversion groove 204 but shoot the side of the PD 206. The horizontal and upward rays L0 and L4 are reflected by the side wall of the PD 206 and are extinguished in vain. The rays L0 and L4 are loss for the PD 206. The reason why the PD 206 inclines to the axial line is that the reflected rays do not return to the LD 205. This is a common technique for light sources making use of laser diodes. {circle around (4)} is an excellent idea since the PLC LD module is based upon the use of the common, inexpensive bottom-incidence type PD.
The Inventors of the present invention, however, think that {circle around (4)} has still some problems. The LD is upside down (epi-down) mounted on the base substrate 200, the emission layer (stripe) 207 is very close to the surface of the substrate. But the emission stripe 207 is still too high in comparison with the bottom of the PD 206. The emission stripe 207 is about 10 xcexcm high from the substrate surface. The rays of the LD have continual distribution which takes the maximum for the just-horizontal ray L0 (211). The strongest L0 ray and the strong lays surrounding L0 cannot go into the bottom of the PD. The PD 206 omits catching the strongest ray. Thus, the monitoring power of the PD of FIG. 20 is about one fifth of the stable, prior art LD module of FIG. 1. The rear light of the LD 205 is not the signal light but the monitoring light. Stronger monitoring light is more desirable for the LD module. The PD which catches only 20% of the LD back light power is unreliable yet for controlling the level of the LD driving power. Another drawback is the skew arrangement to the axial line for preventing the strongest ray from returning to the LD 205.
The PD for monitoring the power of the LD is indispensable for the LD module in the optical communication system. A top incidence type PD will bring about high part cost and high mounting cost, since the PD requires an extra submount for supporting the PD on the side. The end surface incidence type PD or the slanting end surface incidence type PD will raise the cost of manufacturing the PD itself and will reduce the yield. Both types have another drawback of the low efficiency due to the narrow aperture of the PD. Poor photocurrent of the PD decreases the reliability of the system of controlling the LD power. The bottom incidence type PD has a problem of catching only about one fifth of the LD rear light as mentioned just before. The poor monitoring current reduces the reliability also. A purpose of the present invention is to propose a light source (LD module) which enables the PD to generate larger monitoring photocurrent. Another purpose of the present invention is to provide an LD module which facilitates the installation of an LD and a PD on a substrate. A further purpose of the present invention is to provide an LD module endowed with higher reliability through the enhancement of the monitoring current.
The light source (LD module or LD/PD module) of the present invention includes a substrate having a surface, an LD mounted upon a part of the substrate for emitting forward light and rear light, a light waveguide made on a part of the substrate for guiding the forward light emitted from the LD, a path conversion groove formed behind the LD for reflecting the rear light emitted from the LD, a footboard made on the substrate for producing a level higher than the substrate surface and a monitoring PD mounted upon the footboard at the level higher than the substrate surface partially over the path conversion groove for detecting the rear light emitted from the LD.
The rear light emitted from the LD is reflected by the walls of the path conversion groove for guiding the light to the monitoring PD. The role of the footboard is to enhance the light power entering the PD by raising the level of the monitoring. PD. The rise of the PD increases the reflection of the rear light by the walls of the path conversion groove. For example, the footboard for preparing a level higher than the substrate surface is a light waveguide formed on the substrate. Since the light waveguide has a thickness of about 15 xcexcm to 30 xcexcm, the light waveguide can be assigned to the footboard for lifting the PD.
An example of the light source having a footboard as a light waveguide is made by forming a light waveguide on a substrate, eliminating a part of the light waveguide, forming a path conversion groove, mounting an LD upon a naked part without the light waveguide layer and mounting a PD over the path conversion groove on the light waveguide. The lift of the PD reduces the loss of the rear light. More than 50% of the rear light can go into the path conversion groove. The strongest horizontal ray L0 can be reflected and be introduced into the PD for producing extra photocurrent for monitoring due to the enhancement of the PD height.
The general increase of the monitoring light power invites an increase of the photocurrent of the monitoring PD. The enlargement of the PD output raises the preciseness of controlling the LD power for cancelling long term degradation of the LD. The increase of the monitoring PD output enhances the freedom of designing the LD module. The example can exempt this invention from adding an extra step by appropriating the light waveguide which is inherent to the waveguide type LD module as the footboard of the monitoring PD. However, the waveguide is only an example for the footboard. Of course, it is possible to form another footboard than the light waveguide. There is a variety of the material and the shape of the footboard.
The present invention includes a substrate, a light waveguide layer with a core made on the substrate, a path conversion groove perforated along the core on the substrate, an LD bonded on a naked portion of the substrate at an end of the core and a PD fitted over the path conversion groove on the light waveguide layer.
The light waveguide layer is formed for guiding signal light from the LD to an external fiber on the substrate. It is easy to coat all the surface of the substrate with the light waveguide layer which consists of an undercladding, a core and an overcladding. A part of the light waveguide layer or a part of the overcladding layer is facilely eliminated by photolithography. The LD is bonded upon the naked substrate or the naked undercladding at an end of the core. The emission stripe of the LD is laid at the same level as the core in the light waveguide layer. The front light emitted forward from the LD enters the core of the waveguide layer. The rear light emitted back from the LD is reflected by the path conversion groove and is guided into the monitoring PD. The path conversion groove couples the LD and the monitoring PD. The LD lies on the naked substrate or on the undercladding layer. The PD is bonded on the waveguide layer as a footstep. The PD is higher than the LD by the thickness of the waveguide layer or the overcladding layer. Since the PD is higher than LD, the path conversion groove can introduce more than 50% of the rear emitted light from the LD to the PD. An increase of the monitoring photocurrent enhances the preciseness of the system of controlling the LD power and raises the freedom of designing the optical and electric circuits. The light waveguide layer has two roles of guiding the LD signal front light to the external fiber and of lifting the PD as a footstep.
The gist of the present invention is to raise the PD by the waveguide layer as a footstep for increasing the PD monitoring photocurrent. The PD can receive more than 50% of the LD rear light due to the lift of the PD. The function of the path conversion groove, for example, a V-groove is described in detail by referring to FIG. 28 to FIG. 31. In the case of a silicon (001) single crystal, the V-groove can easily be dug by the anisotropic etching.
FIG. 28 is an oblique view of the V-shaped path conversion groove 50. FIG. 29 is the section of the same path conversion groove 50. The top surface is a silicon (001) plane. The V-groove 50 has four slanting walls: a rear slanting wall 57, a front slanting wall 55 and side walls. The crossing line of the side walls is the bottom line 56. The bottom line 56, the front slanting wall 55 and the side walls are insignificant as a mirror for reflecting the LD rear light. xe2x80x9cLxe2x80x9d is the emission point which is the rear end of the emission stripe of the LD. Since the LD is epi-side down bonded on the naked substrate, the height of L is nearly equal to the substrate level. Crossing lines of extensions of four slanting walls and a horizontal plane including light source point L are denoted by ABGK. A, B, G and K are virtual points on the top surface of the substrate. Rigorously speaking, the top corner points of the groove are slightly lower than A, B, G and K. However, it is assumed that the groove is defined by ABGKOC for simplifying the description. Here, two bottom points are denoted by O and C. The substrate should be selected from silicon (Si), GaAs, InP, other semiconductors, ceramics or plastics. Ceramic or plastic substrates allow the path conversion groove to take an arbitrary shape with arbitrary slanting angles. Here, the reflection mode is described for the groove formed on a silicon (001) single crystal substrate as an example.
Then, xcex94KOG is a rear slanting wall 57 M0 (111). xcex94ACB is a front slanting wall 55. Trapezoid ACOK is a left side slanting wall M1 ({overscore (1)}11). Trapezoid BCOG is a right side slanting wall M2 (1{overscore (1)}1). CO is a bottom line [110]. BA is a front end line [{overscore (1)}10]. KG is a rear end line [{overscore (1)}10]. AK is a left side line [110]. BG is a right side line [110]. The most significant mirror is the rear slanting wall M0 (xcex94KOG) (111). The slanting angle ∠BAC or ∠ABC of the side walls is 54.7 degrees in the case of silicon anisotropic etching. The slanting angle of the front wall and the rear wall is also 54.7 degrees. The bottom angle ∠BCA is 70.5 degrees. The light source point L is the middle point of the side AB. xe2x80x9cSxe2x80x9d is a mirror image point of L regarding M1. xe2x80x9cHxe2x80x9d is a mirror image point of L with regard to M2 in FIG. 29. H and S are deemed as virtual light source points extended by M1 and M2. Namely, there are three light source points L, H and S.
The beams starting from L, H and S should rigorously be traced on the basis of the beam tracing calculation method. A simple geometric optics can clarify the advantage of the lift of the PD higher than the LD for reinforcing the monitoring power.
FIG. 30 is a section of the groove extended symmetrically with regard to the rear slanting mirror M0 which plays the main role for transferring the LD power to the PD when the level difference h between LD and PD is 0 (h=0). The prior art {circle around (4)} belongs to FIG. 30. Lxe2x80x2 is a mirror image point of L regarding M0. Hxe2x80x2 and Sxe2x80x2 are mirror image points of H and S regarding M0. Hatched PDxe2x80x2 is a mirror image of the PD 52 regarding M0. The rays starting from the sources L, H and S and reflected by M0 can be replaced by the straight rays starting from the virtual sources Lxe2x80x2, Hxe2x80x2 and Sxe2x80x2.
The rays are refracted at the boundary of the PD and the outer resin in accordance with Snell""s law. The influence of the refraction should be conveniently eliminated by reducing the PD size. The PD substrate has a refractive index n1 and the outer resin (e.g., silicone resin) has a refractive index n2. The ratio n1/n2 is denoted by n (=n1/n2). The height of the PD is reduced to 1/n to the bottom. The bold line shows the reduced virtual PD which allows the boundary to replace the refracted rays by straight lines.
In FIG. 30 (h=0, prior art {circle around (4)}, Lxe2x80x2 emits three virtual rays L0, L1 and L4. Since h=0, the strongest L0 is disturbed by the side of the PDxe2x80x2. The upward ray L4 is also shielded by the side of the PDxe2x80x2. The downward ray L1 can arrive at the sensing region 54 of the PD. H4 and S4 from the side virtual sources Hxe2x80x2 and Sxe2x80x2 reach the PD. But the light from the side virtual sources is weak. Since the main ray L0 is rejected by the PD, the light power which can arrive at the sensing region 54 is far less than 50% of the LD rear light.
In FIG. 31 (h greater than 0, present invention raising the PD), Lxe2x80x2 emits five virtual rays L0, L2, L3, L4 and L5. Since h greater than 0, the strongest L0 is not disturbed by the side of the PDxe2x80x2. The PD monitoring power is enhanced by the incidence of the horizontal ray L0. The slightly upward ray L4 is also absorbed by the sensing region 54 of the PD, because the PD recedes upward. The downward ray L2 and L3 can also arrive at the light receiving region 54 of the PD. Since the strongest main ray L0 is accepted by the PD, the light power which can arrive at the sensing region 54 is more than 50% of the LD rear light. The upheaval of the PD increases the LD power attaining to the monitoring PD. The increase of the monitoring power enables the module to control the LD power more precisely. The freedom of designing the optical parts and the electric circuits is also enhanced.